A predominant number of color picture tubes in use today have line screens and shadow masks that include slit-shaped apertures. The apertures are aligned in columns, and the adjacent apertures in each column are separated from each other by webs or tie bars in the mask. Such tie bars, which define the vertical pitch (a.sub.v) of the mask, are essential in the mask to maintain its integrity when it is formed into a dome-shaped contour which somewhat parallels the contour of the interior of a viewing faceplate of the tube. In earlier tubes of this type, the separations between adjacent aperture column centerlines along the major axis, or horizontal pitch (a.sub.H), was held constant from center-to-edge of the mask. However, some later tubes of this type included a shadow mask with increased curvature and incorporated a shadow mask with an aperture column pitch variation as taught in U.S. Pat. No. 4,136,300, issued to A. M. Morrell on Jan. 23, 1979. In such later tubes, the pitch between centerlines of adjacent aperture columns increased from center-to-edge of the mask. This increase varied along the major axis generally as the square of the distance from the minor axis. In yet later tubes, a shadow mask aperture column-to-column pitch that varied along the major axis as the fourth power of the distance from the minor axis, such as taught in U.S. Pat. No. 4,583,022, issued to W. D. Masterton on Apr. 15, 1986, was used.
A problem that may occur during operation of color picture tubes is video aliasing. Video aliasing is an artifact, defect or distortion in a video picture usually occurring when video signals contain high frequencies. Aliasing can occur with both analog and digital video signals. With analog video, aliasing is usually caused by interference between two frequencies, such as might occur between high luminance frequencies and the horizontal spatial frequency due to the horizontal periodicity of the shadow mask of the tube. This aliasing appears as moire or herringbone patterns. With digital video, aliasing is caused by insufficient sampling or poor filtering of the digital signal, and shows up as jagged edges on diagonal lines and fluctuations in picture detail.
In video systems, aliasing will result when an image is sampled that contains frequency components above the Nyquist limit for the sampling rate. The Nyquist frequency (f.sub.N) is the maximum frequency that a shadow mask can theoretically represent. The easiest solution to this aliasing problem is to increase the Nyquist frequency of the tube by decreasing the horizontal screen pitch. However, this solution has both cost and production limits. Another solution is to design an electron gun which has a larger beam spot at low current. However, a larger beam spot may degrade tube resolution to an unacceptable extent. Therefore, there is a need for solutions to the video aliasing problem which do not have these limitations.